Updated Saturday, August 11, 10:06 a.m. EDT -- The Obama campaign released this statement in reaction to Ryan's selection: "In naming Congressman Paul Ryan, Mitt Romney has chosen a leader of the House Republicans who shares his commitment to the flawed theory that new budget-busting tax cuts for the wealthy, while placing greater burdens on the middle class and seniors, will somehow deliver a stronger economy. The architect of the radical Republican House budget, Ryan, like Romney, proposed an additional $250,000 tax cut for millionaires, and deep cuts in education from Head Start to college aid. His plan also would end Medicare as we know it by turning it into a voucher system, shifting thousands of dollars in health care costs to seniors. As a member of Congress, Ryan rubber-stamped the reckless Bush economic policies that exploded our deficit and crashed our economy. Now the Romney-Ryan ticket would take us back by repeating the same, catastrophic mistakes.”

Updated Saturday, August 11, 9:55 a.m. EDT -- In a stump speech that warned America is going down the wrong path, Paul Ryan boiled down the essence of his partnership with Romney to this: "I believe that my record of getting things done in Congress will be a very helpful complement to Governor Romney's executive and private sector success outside of Washington."

Updated Saturday, August 11, 9:35 a.m. EDT -- Perhaps swept up in the excitement of the moment, Romney introduced Paul Ryan by saying, "Join me in welcoming the next president of the United States, Paul Ryan." Later on, Romney quipped, "Everyone now and then I'm known to make a mistake. But I did not make a mistake with this guy." It's worth noting that then-candidate Barack Obama did the same thing with Joe Biden back in 2008.

Updated Saturday, August 11, 9:30 a.m. EDT -- In a move clearly designed to shore up the Republican base, Mitt Romney has chosen Rep. Paul Ryan (R-Wisc.) as his running mate. Romney made the announcement Saturday morning in Norfolk, Va. "His leadership begins with character and values," said Romney. He praised Ryan's belief in "the worth and dignity of every human life," and said, "With energy and vision, Paul Ryan has become an intellectual leader of the Republican Party. He understands the fiscal challenges facing America: our exploding deficits and crushing debt, and the fiscal catastophe that awaits us if we don't change course."

Earlier:

NBC News and CNN, citing sources close to the Romney campaign, are reporting that House Budget Committee Chairman Paul Ryan will be named Mitt Romney's vice presidential choice when the selection is announced Saturday morning.

Romney is set to reveal his running mate at the U.S.S. Wisconsin, before setting out on a tour of key swing states to highlight his economic plans for the middle class.

In an interview with NBC on Thursday, Romney said he was looking for someone with " a strength of character" and "a vision for the country that adds something to the political discourse about the direction of the country." ...

Ryan has proposed to dramatically change Medicare, the government-run health insurance program for seniors, and overhaul other entitlement programs. His budget plan has been widely criticized by President Obama and his fellow Democrats, who contend it would end Medicare as we know it.

Why Ryan? The Daily Beast's Howard Kurtz characterizes the reported choice as effort by Romney to "double down on conservative ideology" in a way that would eliminate his campaign's "ideological fuzziness."

In a single stroke, Romney energizes the right wing of his party by picking the man who many Republicans revere as the intellectual leader of the party's drive to shrink government. But he also hands President Obama and the Democrats a titanic target with a paper trail, given Ryan's budget-slashing plan, approved by the House, that would turn Medicare into a voucher program ...

The Ryan pick will help Romney enormously in one key respect. The choice emphatically changes the subject at a time when the former Massachusetts governor has spent weeks on the defensive, on issues ranging from his failure to release more tax returns to his tenure at Bain Capital to the lack of a defined message. Romney has slipped behind Obama by as much as 9 points in national polls, as well as in a number of key swing states.

As of Saturday, Romney's campaign will no longer suffer from ideological fuzziness. The question is whether his new running mate, while exciting the base, will help Romney win over crucial independent voters -- or scare them away.